This invention relates to a dynamic random access memory with a self-refreshing function.
As a large capacity memory device, a dynamic random access memory (DRAM) in which one MOS transistor and one capacitor compose each memory cell is advantageous and general.
In this type of memory device, an electric charge is stored in the capacitor, and the written information is judged to be a binary 0 or 1 depending on whether this charge is present or absent. However when allowed stand idle for a long period of time, the stored charge decreases due to the leakage current of the capacitor, and the written information may be lost. Therefore, before the stored charge has decreased below a predetermined level, the information is read out from the capacitor, and the same information is then written again into the capacitor in order to prevent the information from disappearance. This operation is called refreshing.
However, in conventional dynamic random access memory devices, it is indispensable to keep refreshing even when the information is neither read out from the capacitor nor written into the capacitor by supplying address signals and clock signals to the memory device. Such a refreshing function is performed by external circuits, and therefore, the memory system tends to be complicated.
Recently, in order to simplify the memory system, it has been proposed to perform refreshing by internal circuits which are fabricated into the memory device (for example, Nikkei Electronics, No. 215, p. 167, 1979; No. 418, p. 167, 1987). This system is called self-refreshing function.
A conventional self-refreshing function of dynamic random access memory device is described below.
A block diagram of a conventional dynamic random access memory device is shown in FIG. 7, in which a memory cell matrix 21 has plurality of memory cells of m rows.times.n columns. A row line selection circuit 22 selects any one of the row lines of the memory cell matrix 21. A sense-refreshing amplifier 23 refreshes the memory cells corresponding to the row line selected by the row line selection circuit 22. A self-refreshing clock generator 24 is self-activated in a predetermined period, and generates a series of self-refreshing clock pulses. A control clock generator 25 generates a series of control clock pulses in synchronism with the self-refreshing clock pulses. A refreshing address counter 26 stores the refreshing address.
The self-refreshing operation of the dynamic random access memory device shown in FIG. 7 is as follows.
First, by the control clock synchronized with the self-refreshing clock, memory cells corresponding to the row line address specified by the refreshing address counter 26 and selected by the row line selection circuit 22 are refreshed. Afterwards, the refreshing address counter 26 is incremented by 1 bit within 1 refreshing period. When this operation is repeated continuously, all memory cells are completely refreshed by executing self-refreshing in m periods in a memory device having a memory cell matrix of m rows.times.n columns.
The self-refreshing clock generator 24 generates a series of clock pulses by dividing the oscillator output having a predetermined period. Accordingly, the self-refreshing period, once determined, does not change significantly although it does vary somewhat depending on the temperature condition or the deviation of a supply voltage.
The mean current consumption during self-refreshing is nearly inversely proportional to the period of the self-refreshing clock. In order to decrease the current consumption, it is better to set a longer period of the self-refreshing clock. However, if this period is set longer than the value of dividing the pause time T.sub.H to the memory cell by the number of row lines m of the memory cell matrix, it is impossible to refresh all memory cells within the pause time T.sub.H. Furthermore, the pause time T.sub.H decreases to about half when the ambient temperature is raised by 10.degree. C., and it also varies significantly depending on the manufacturing process condition. Therefore, in order to execute the self-refreshing operation correctly, it is necessary to set the self-refreshing clock period to a shorter period with a sufficient margin of safety, which makes it difficult to decrease the current consumption.
It is hence difficult, in the dynamic random access memories having the conventional self-refreshing function, to achieve the mutually contradictory purposes of decreasing the current consumption during self-refreshing and improving the precision of the self-refreshing operation.